Liquid siphon tank manifolding is a common method used at retail fueling sites and elsewhere to increase the total storage capacity available for any one fuel type. Reconciliation methods used for tracking tank system integrity require highly accurate tank calibration charts to minimize discrepancies when reconciling dispensing meter amounts against tank readings. A tank chart, also known as a tank strap chart, is used to convert tank fuel height readings, in inches, to volume, in gallons, which are needed to compare to the metered volumes which are also in gallons. Currently, no convenient method exists to provide the highly accurate tank calibration charts needed for reconciliation when tanks are manifolded together.
Tank manifolding is especially convenient in the United States since the recent changeover from leaded fuel to unleaded fuel left site owners with, typically, three tanks in the ground to handle only two types of fuel. Previously, separate tanks were needed for low grade leaded, low grade unleaded, and high grade (typically unleaded). Now only low grade and high grade unleaded fuels are needed. Midgrades can be blended from the high and low grade unleaded fuels. Since low grade generally out-sells high grade by a two to one factor, the extra tank can be manifolded to the low grade tank to handle the demand by doubling the storage capacity. This presents a problem for current methods of tank calibration which can only be used with single tanks.
Known single tank calibration techniques measure the drop in tank fuel height during dispensing periods and compare the volume changes associated with the height drop to the amount of fuel dispensed through dispensing meters during these periods. Actual gallons per inch contained in the tank can be identified at various tank heights by using the metered volume versus tank height drop ratios, or (gallons per inch=dV/dh=(metered volume in gallons/height drop in inches) at height h). Since dV/dh varies with tank height, h, these data are accumulated over a range of fuel heights during normal tank activity. When adequate data are accumulated, a tank calibration may be performed by adjusting the existing tank chart, or creating a new tank chart. Various techniques are available to use the data to perform this function.
The data available from manifolded tank systems for calibration purposes includes the same metered volumes and height drops, but in this case, height drops are available from both tanks. This causes the problem of choosing which tank to attribute the amounts dispensed. All of the fuel does not come from just one tank. In fact, both tanks supply the fuel due to the manifolded siphon which eventually allows the levels to even out between the tanks. This takes time due to the fast rates of dispensing relative to the limited size of the siphoning action. However, the time available to take the fuel height readings typically does not allow for a long enough settling period to ensure that the fuel heights are equal between the tanks. Even if settled or static heights were available, it still cannot be determined how much metered fuel to attribute to each tank.
The present invention is directed at providing a solution to the above problem.